Integrated circuit frequency generator

ABSTRACT

An integrated circuit frequency generator is disclosed. In some embodiments, the frequency generator comprises an electronic oscillator configured to generate an oscillator frequency and calibration circuitry configured to periodically calibrate the electronic oscillator with respect to a reference frequency source. When a primary power source is unavailable, an output frequency is generated from the oscillator frequency, and the reference frequency source is powered-on only during calibration cycles.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/453,974 entitled INTEGRATED CIRCUIT FREQUENCY GENERATORfiled Apr. 23, 2012 which is incorporated herein by reference for allpurposes, which is a continuation of U.S. Pat. No. 8,183,937 entitledINTEGRATED CIRCUIT FREQUENCY GENERATOR filed Feb. 12, 2010 which isincorporated herein by reference for all purposes and which claimspriority to U.S. Provisional Patent Application No. 61/207,647 entitledGENERATING MULTIPLE CLOCK FREQUENCIES USING A SINGLE CRYSTAL filed Feb.13, 2009 and U.S. Provisional Patent Application No. 61/276,184 entitledFREQUENCY GENERATOR filed Sep. 8, 2009, which are incorporated herein byreference for all purposes.

BACKGROUND OF THE INVENTION

Quartz crystals are typically employed as reference frequency sources inelectronic systems due to their outstanding performance characteristics.Most electronic systems include at least one high frequency (e.g., MHz)crystal. High frequency crystals provide excellent phase noise,stability, and aging characteristics but have the drawback of consumingmore power than lower frequency crystals. Thus, a high frequency crystalmay be employed in a system when power consumption is not constrained.However, when power conservation is critical, use of a high frequencycrystal may be unacceptable. Consequently, most systems also include atleast one low frequency (e.g., 32.768 kHz) crystal for low powerconsumption needs. The low frequency crystal compromises phase noise,some stability, and some aging characteristics in return for asubstantially lower power consumption.

Existing electronic systems typically include a plurality of crystalsfrom which to generate required system frequencies under variousoperating conditions. For example, a typical notebook computer mayinclude as many as nine crystal frequency sources. Although thereference frequency generated by a single high frequency crystal can beused to generate any required system frequency, the high frequencycrystal consumes too much power and thus cannot be employed when powerconsumption is critical. Although the reference frequency generated by asingle low frequency crystal can be used to generate any required systemfrequency, the low frequency crystal introduces unacceptable phasenoise, e.g., when coupled to a phase-locked loop, and thus in many casescannot be employed to generate other required system frequencies. Noviable techniques exist for eliminating the need for a plurality ofcrystal frequency sources in electronic systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1A illustrates a high level block diagram of an embodiment of afrequency generator integrated circuit.

FIG. 1B illustrates a high level functional block diagram of thecomponents comprising an embodiment of a frequency generator integratedcircuit.

FIG. 2 illustrates an embodiment of a process for generating a clockfrequency.

FIG. 3 illustrates an embodiment of a process for calibrating anoscillator.

FIG. 4 illustrates an embodiment of regulator circuitry.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims,and the invention encompasses numerous alternatives, modifications, andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example, andthe invention may be practiced according to the claims without some orall of these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

A micro-power integrated circuit frequency generator calibrated with asingle high frequency quartz crystal is disclosed. The frequencygenerator described herein may be employed to generate multiple clockfrequencies using semiconductor technology and one crystal rather than aplurality of crystals in systems that require a plurality of clocksignals of various frequencies. Elimination of all but a single crystalfrequency source in a system provides several benefits. For example, useof a single crystal with the disclosed integrated circuit frequencygenerator results in a lower cost in systems that would otherwise use aplurality of crystal frequency sources. Moreover, since lower frequencycrystals are electro-mechanical devices that age and deteriorate overtime, reducing the number of crystals in a system to just a singlehigher frequency crystal results in more robust aging characteristicsand higher reliability due to fewer crystals. Furthermore, sincecrystals are relatively large devices, use of a single crystal with thedisclosed integrated circuit frequency generator rather than a pluralityof crystals results in less board space being consumed for clockfrequency generation.

As further described herein, the frequency generator integrated circuitmay be employed to generate low frequencies using a high frequencycrystal source with high accuracy, over wide temperature ranges, and atvery low power consumption. For example, a reference frequency generatedby a high frequency crystal source in the MHz range may be employed bythe frequency generator to generate a 32.768 kHz clock having anaccuracy on the order of +/−20 parts per million of error and with acurrent consumption on the order of 1-10 to depending on short termaccuracy considerations. In existing systems, the 32.768 kHz frequencyis typically generated using a dedicated 32.768 kHz crystal to maintainhigh accuracy and low power consumption. Generating the 32.768 kHzfrequency with high accuracy and low power consumption is essentialbecause this frequency is employed in real time clock (RTC) applicationsin most computer and consumer electronic products. Very low powerconsumption is essential because most systems that include RTC functionsmaintain timekeeping even when the systems are in a sleep state orpowered off and thus need to maintain these functions under backupbattery power. The frequency generator integrated circuit describedherein eliminates the need for a separate 32.768 kHz crystal while stillproviding at least comparable and in most cases improved performance.

FIGS. 1A-1B illustrate high level functional block diagrams ofembodiments of a frequency generator integrated circuit that generatesone or more frequencies using a single reference frequency source.Although a 24.576 MHz crystal reference frequency source and aparticular integrated circuit configuration to generate variousfrequencies including a 32.768 kHz frequency are described in some ofthe given examples, the disclosed techniques may be similarly employedwith respect to any other appropriate reference frequency source andintegrated circuit configuration to generate any one or more desiredclock frequencies.

FIG. 1A illustrates a high level block diagram of an embodiment of afrequency generator integrated circuit. In the given example, frequencygenerator integrated circuit 100 employs reference frequency source 102to generate one or more output clock frequencies. As depicted, alloutput frequencies are based on the reference frequency generated byreference frequency source 102. In different modes of operation,frequency generator 100 may be powered by different power sources.During a sleep and/or power-off mode, frequency generator 100 is poweredby a backup power source such as a coin cell battery, which is indicatedin the given figure by power source 104. During an active and/orpower-on mode, frequency generator 100 is powered by a main power sourcesuch as a wall power unit or main battery, which is indicated in thegiven figure by power source 106.

FIG. 1B illustrates a high level functional block diagram of thecomponents comprising an embodiment of frequency generator 100. In thegiven example, only the components and/or connections relevant todescribing the functionality of frequency generator 100 are depicted. Invarious embodiments, frequency generator 100 may comprise any otherand/or different components and/or configuration to provide thedescribed functionality. Frequency generator 100 may operate in eitherpower-off mode or power-on mode. In power-off mode, non-essentialcircuitry is powered off, and frequency generator 100 only generates theone or more frequencies needed by an associated system when powered-off,such as the 32.768 kHz RTC frequency, using back-up battery power. Inpower-on mode, frequency generator 100 generates various frequenciesneeded by an associated system when powered-on, e.g., variousfrequencies in the range of 32.768 kHz-25 MHz in the given example. Asdepicted, frequency generator 100 is powered by a 1.2 V back-up powersource 104 in power-off mode and a 2.5 V main power source 106 inpower-on mode. In FIG. 1B, the reference frequency source comprises a24.576 MHz quartz crystal 102.

In power-off mode, the 32.768 kHz output frequency is generated usingelectronic oscillator 108. However, compared to a crystal frequencysource, an electronic oscillator is relatively unstable, and itsaccuracy drifts over time. In order to ensure a 32.768 kHz output havingat least a prescribed accuracy, crystal 102 is periodically pulsepowered on so that oscillator 108 can be periodically calibrated usingthe highly accurate reference frequency output by crystal 102.Periodically powering crystal 102 to provide a highly accurate referencefrequency via which oscillator 108 can be calibrated and powering offcrystal 102 until the next calibration cycle while in power-off moderesults in the 32.768 kHz output frequency being generated with very lowpower consumption and on average with a very high accuracy sinceoscillator 108 is calibrated to track the accuracy of crystal 102. Inpower-on mode, power conservation is not as critical, and crystal 102 ispowered on so that various frequencies needed by the system can begenerated using the highly accurate reference frequency output bycrystal 102. As a result, rather than using relatively lower power butunstable oscillator 108 to generate the 32.768 kHz output frequency inpower-on mode, frequency divider 110 is instead employed to divide thereference frequency output by crystal 102 to produce the 32.768 kHzoutput frequency. Thus, as described in further detail below, frequencygenerator 100 is configured to operate in three states—power-off freerun mode, power-off frequency calibration mode, and power-on mode.

In power-off mode, frequency generator 100 is powered by back-up powersource 104. Back-up power source 104 comprises a backup battery such asa coin cell battery. In this mode, the main power source of anassociated system is off or unavailable, and power conservation iscritical. Certain system functions, such as RTC functions, need to beoperational even when the system is off; and the clock frequenciesrequired for these functions, such as 32.768 kHz, need to be generatedby frequency generator 100. Frequency generator 100 employs anelectronic oscillator 108 to generate an output clock frequency in thismode. Oscillator 108 may comprise any appropriate electronic oscillatoror equivalent electronic circuit. For example, oscillator 108 maycomprise a ring oscillator, a DCO (digitally-controlled oscillator), aVCO (voltage-controlled oscillator), an LC VCO, etc. An oscillatorhaving any desired configuration and characteristics may be selectedbased on the needs of the application. In most cases, a low power (e.g.,in the μW range) oscillator is desirable since power conservation iscritical in power-off mode. Frequency generator 100 of FIG. 1B includesa 32.768 kHz oscillator 108 to generate a 32.768 kHz output frequency.In other embodiments, an oscillator having a different frequency than adesired output frequency may be employed in combination with a frequencymultiplier and/or frequency divider or other equivalent circuitry togenerate a prescribed output frequency. For example, a 131.072 kHzoscillator and divide-by-four frequency divider may be employed togenerate a 32.768 kHz output frequency. Since the size of an oscillatoris somewhat inversely proportional to frequency, it may be desirable touse an oscillator having a higher frequency and a frequency divider toproduce a desired output frequency to achieve a smaller board area. Forany desired output frequency, an optimal frequency for the oscillatormay be selected based on process geometry. In some embodiments,oscillator 108 is completely integrated into frequency generatorintegrated circuit 100. In other embodiments, oscillator 108 may not becompletely integrated into frequency generator integrated circuit 100and may include one or more external components such as resistors,capacitors, transistors, etc., that contribute to the function of theoscillator.

In power-off free run mode, only oscillator 108 and other essentialcircuitry are powered by back-up power source 104. A free runningoscillator, however, is inherently a relatively unstable and inaccuratedevice, typically having an error on the order of +/−20000 ppm andhigher. Various factors contribute to the frequency error. An oscillatoris highly sensitive to thermal variations. For example, several hundredppm of frequency error typically results for each degree Celsius changein temperature of an associated integrated circuit. Other factors thatcontribute to frequency error include process variations duringmanufacturing and mechanical stresses during the mounting of anassociated integrated circuit to a printed circuit board. The largeerror of an untrimmed electronic oscillator cannot be tolerated by manysystem functions, including RTC functions. In order for oscillator 108and an associated output frequency to have an acceptable level ofaccuracy (e.g., on the order of +/−20 ppm of error), dynamic calibrationis periodically performed to trim oscillator 108.

Frequency generator 100 includes timer 112 which facilitates togglingbetween power-off free run and frequency calibration modes. In someembodiments, calibration timer 112 comprises a counter. In some suchcases, timer 112 counts a prescribed number of pulses output byoscillator 108 between calibration cycles and may be programmed orconfigured for any desired calibration interval or period. More frequentcalibration of oscillator 108 results in higher power consumption butgreater short term accuracy of the output frequency associated withoscillator 108 while less frequent calibration of oscillator 108 resultsin lower power consumption but also lower short term accuracy of theoutput frequency associated with oscillator 108. However, despite theselected calibration interval, on average the output frequencyassociated with oscillator 108 has an accuracy that is comparable tothat of a crystal since oscillator 108 is periodically calibrated usingthe relatively accurate (e.g., +/−20 ppm of error) reference frequencygenerated by crystal 102 as further described below. In the example ofFIG. 1B, one of three calibration intervals may be selected for timer112: every 32768*1 pulses or equivalently every second for a 32.768 kHzoscillator, every 32768*60 pulses or equivalently every minute for a32.768 kHz oscillator, or every 32768*120 pulses or equivalently everytwo minutes for a 32.768 kHz oscillator. In other embodiments, timer 112may be programmed or configured for any desired calibration interval.After a prescribed number of pulses or time period since the lastcalibration cycle, e.g., as determined by timer 112, frequency generator100 switches from power-off free run mode to power-off frequencycalibration mode while continuing to output the 32.768 kHz frequencygenerated by oscillator 108. In the given example, toggling frompower-off free run mode to power-off frequency calibration mode isachieved by closing switch 114, which powers on crystal 102 and othercalibration circuitry with back-up power source 104.

In power-off frequency calibration mode, crystal 102 is briefly turnedon to calibrate oscillator 108. Closing switch 114 powers on oscillator116, which drives crystal 102. Oscillator 116 may comprise anyappropriate oscillator or other circuit that can be used as a driver forcrystal 102. After sufficient time (e.g., ˜0.5 ms) has lapsed forcrystal 102 to settle to a steady state frequency after turn-on, thereference frequency generated by crystal 102 is used to calibrateoscillator 108. In the given example, counter 118 counts a prescribednumber of pulses of oscillator 108 while pulse comparator logic 120(which in some embodiments also includes a counter) counts the number ofpulses output by crystal 102 during that period. For example, pulsecomparator logic 120 may be configured to count the number of pulses of24.576 MHz crystal 102 that transpire during the period of time counter118 counts 264 pulses of 32.768 kHz oscillator 108, which isapproximately equivalent to 8 ms. Ideally, there should exist a fixedration of 750 24.576 MHz pulses for each 32.768 kHz pulse. Pulsecomparator logic 120 is able to determine whether the current actualfrequency of oscillator 108 is too high or too low based on the numberof pulses of the 24.576 MHz crystal 102 counted during the period oftime counter 118 counts 264 pulses of oscillator 108. Based on thedetermination, pulse comparator logic 120 appropriately trims oscillator108 to correct the frequency error of oscillator 108. Any of a varietyof trimming techniques known in the art may be employed to trimoscillator 108. In the given example, pulse comparator logic 120 adjustsa capacitor array 122 connected to oscillator 108 to correct thefrequency error of oscillator 108. In this example, adding morecapacitance to oscillator 108 decreases its frequency while removingcapacitance from oscillator 108 increases its frequency. One or morefurther iterations of comparing the pulses of oscillator 108 withrespect to the pulses of crystal 102 and adjusting capacitor array 122,as applicable, may be performed until oscillator 108 closely tracks theaccuracy of crystal 102. Once the calibration process has completed,switch 114 is opened, which powers off crystal 102 as well as the othercalibration circuitry and returns frequency generator 100 back topower-off free run mode, which is the lowest power consumption mode.

In the example described above, 264 pulses of oscillator 108 are countedby counter 118 and employed by pulse comparator logic 120 to determinethe frequency error of oscillator 108. In other embodiments, any numberof pulses of oscillator 108 may be used to determine the frequency errorof the oscillator, and counter 118 and pulse comparator logic 120 may beaccordingly configured. In some cases, the number of pulses ofoscillator 108 used to make a frequency error determination may be basedon a desired accuracy resolution. A better accuracy is achievable usingmore pulses; however, fewer pulses may be employed if compromisingaccuracy at least to a certain extent is tolerable. For example, asingle pulse of oscillator 108 may be sufficient to calibrate it toapproximately +/−1000 ppm of error. Comparing fewer pulses of oscillator108 during each iteration of a calibration cycle results in less powerconsumption per iteration but at the expense of accuracy resolution. Theaccuracy resolution also depends on the number of iterations performedduring each calibration cycle. In some embodiments, a prescribed numberof iterations may be performed during each calibration cycle.Alternatively, in some embodiments, as many iterations needed to achievea prescribed accuracy may be performed during each calibration cycle.Fewer iterations during a calibration cycle result in lower powerconsumption but at the expense of less accuracy.

As described, the accuracy of the output frequency associated withoscillator 108 and the power consumed to generate it during power-offmode depend on various parameters such as the type of oscillator 108,the number of pulses of oscillator 108 compared against the referencefrequency generated by crystal 102 during an iteration of a calibrationcycle, the number of iterations for adjusting the frequency ofoscillator 108 per calibration cycle, the frequency of the calibrationcycles, etc. Such parameters may be tunable and selected based on theneeds of an associated system. A higher accuracy is achievable withlonger and/or more frequent calibration cycles but at the expense ofmore power consumption. A lower power consumption is achievable withshorter and/or less frequent calibration cycles but at the expense ofreduced accuracy. Regardless, frequency generator 100 may be employed togenerate a clock frequency while in power-off mode having an accuracycomparable to that of a quartz crystal (e.g., +/−20 ppm of error) andwith a very low power consumption (e.g., a few μW). As described withrespect to the example of FIG. 1B, frequency generator 100 generates avery low power 32.768 kHz clock frequency having an accuracy comparableto that of the reference frequency generated by high frequency crystal102. The power consumption of high frequency crystal 102 duringpower-off mode is drastically reduced by only intermittently orperiodically turning it on rather than keeping it on continuously.

Although a particular technique and circuitry for calibrating oscillator108 with respect to crystal 102 is depicted by and described withrespect to FIG. 1B, in various embodiments, any other appropriatetechnique or circuitry may be employed. For example, in someembodiments, a frequency-locked loop (FLL) may be employed to calibrateoscillator 108. In some such cases, sigma-delta modulation may beemployed with the FLL to control oscillator 108. For example, asigma-delta modulator may be inserted between a conventional FLL controloutput and the oscillator that it controls. In some embodiments, thesigma-delta modulator creates frequent updates to the oscillator basedon a slow-changing FLL output, thereby achieving suppression ofquantization error without requiring a fast-updating FLL.

In power-on mode, frequency generator 100 is powered by main powersource 106. In various embodiments, any appropriate switching techniqueto switch between power source 104 and power source 106 may be employedwhen switching between power-off and power-on modes. In some cases, aswitching technique that is compatible with multiple power sources ofpossibly differing values or voltages is employed. In power-on mode,power conservation is not as critical. However, it is still important togenerate accurate clock frequencies. In power-on mode, crystal 102 isturned-on so that the reference frequency generated by crystal 102 canbe used to generate various clock frequencies needed by an associatedsystem. In various embodiments, frequency multipliers, frequencydividers, frequency-locked loops, phase-locked loops, or any otherappropriate circuitry may be employed in power-on mode to generatevarious clock frequencies using the reference frequency of crystal 102.In some embodiments, frequency generator 100 employs fractional-ndividers or synthesizers, e.g., because simple metal or non-volatilememory programming can be employed with such devices to achieve a properdivide ratio for any reference frequency to generate a desired outputfrequency.

In FIG. 1B, the 24.576 MHz reference frequency generated by crystal 102is divided by 750 by frequency divider 110 to generate the 32.768 kHzoutput frequency in power-on mode. Low power electronic oscillators suchas oscillator 108 are extremely sensitive to thermal variations and tendto drift and perform poorly in an environment in which temperaturegradients occur. Consequently, oscillator 108 may not be employed togenerate the 32.768 kHz output frequency in power-on mode in someembodiments due to the thermal variations arising from the activity ofvarious other devices on an associated die and/or circuit in power-onmode. However, in some embodiments, oscillator 108 may be employed togenerate the 32.768 kHz output frequency in power-on mode if thecalibration rate is increased to account for the additional thermalrelated error. Because power conservation is not as critical in power-onmode, the 32.768 kHz output frequency may be generated directly bydividing the 24.576 MHz frequency produced by crystal 102 or thecalibration rate may be increased. In the example of FIG. 1B, the outputof multiplexor 124 switches from the output of oscillator 108 inpower-off mode to the output of frequency divider 110 in power-on mode.Any appropriate phase alignment techniques known in the art may beemployed to ensure a seamless 32.768 kHz output when switching sourcesof the output frequency.

In some embodiments, frequency generator 100 may be configured togenerate a single output frequency (e.g., 32.768 kHz) in all operationalmodes. In the embodiment of FIG. 1B, frequency generator 100 is alsoconfigured to output 25 MHz, 14.318 MHz, and 24.576 MHz clockfrequencies in power-on mode. As depicted, phase-locked loops 126 and128 are employed in power-on mode to generate output frequencies of 25MHz and 14.318 MHz, respectively, from the 24.576 MHz referencefrequency of crystal 102. In the given example, frequency generator 100integrates phase-locked loops 126 and 128. In other embodiments, suchcircuitry may not be integrated in frequency generator 100 and mayinstead comprise external components. In various embodiments, frequencygenerator 100 may be implemented differently than the example providedin FIG. 1B and may be configured to generate or output any number ofcopies of any one or more frequencies.

As described, the thermal induced frequency error of electronicoscillator 108 due to power-on temperature gradients is circumvented byswitching from oscillator 108 to simply dividing the reference frequencyof crystal 102 by divider 110 during power-on mode to produce the 32.768kHz output frequency. Temperature gradients also occur when switchingfrom power-on mode to power-off mode. A sudden cooling across anassociated die and/or circuit when entering power-off mode introduces atransient error in oscillator 108 at least until a steady statepower-off temperature is reached. Such errors may accumulate into asystematic offset or bias over time, especially if an associated systemis frequently powered on and off. In various embodiments, the bias maybe corrected or compensated using any appropriate techniques. In somecases, for instance, the bias is at least in part removed or averagedout by calibrating the oscillator very frequently at least for a shortperiod of time after entering power-off mode as the associated systemcools down. For example, oscillator 108 may be calibrated every 1/10 or1/15 of a second for the first few minutes in power-off mode. In somecases, even though the output of oscillator 108 is not employed inpower-on mode, the frequency error of the oscillator resulting from thetemperature surge at turn-on is measured and stored and later subtractedat power-off to compensate for cool down.

FIG. 1B illustrates one embodiment of the manner in which a frequencygenerator integrated circuit may at least in part be implemented. Inother embodiments, frequency generator 100 may be implemented and/orconfigured in any manner such that it provides the basic functionalityof using a single high accuracy reference frequency source to generateany output clock frequency having a comparable accuracy as the referencefrequency source and at a very low power consumption at least inpower-off mode.

FIG. 2 illustrates an embodiment of a process for generating a clockfrequency having high accuracy and very low power consumption inpower-off mode using a single reference frequency source. In someembodiments, process 200 is employed by frequency generator 100 of FIGS.1A-1B. In some embodiments, process 200 is employed to generate the32.768 kHz clock frequency in various operational modes. If in process200 it is determined that the mode is power-on mode, the clock frequencyis generated from the reference frequency source at 202. In some cases,for example, the reference frequency is divided using a frequencydivider to generate the clock frequency. If in process 200 it isdetermined that the mode is power-off mode, the clock frequency isgenerated using a low power electronic oscillator at 204. At 206, it isdetermined whether to calibrate the oscillator. Since the oscillator isrelatively unstable, it periodically needs to be trimmed so that theclock frequency generated using the oscillator has at least a prescribedaccuracy. If it is determined at 206 to calibrate the oscillator,calibration circuitry (including the reference frequency source) ispowered-on at 208. In some embodiments, 208 includes waiting for thereference frequency source to settle after turn-on. At 210, theoscillator frequency is calibrated with respect to the referencefrequency. For example, 210 includes trimming the oscillator so that itsaccuracy closely tracks the accuracy of the reference frequency source.Once calibration of the oscillator is complete at 210, the calibrationcircuitry (including the reference frequency source) is powered off at212 until the next calibration cycle, and process 200 continues at 206.In power-off mode, the clock frequency is generated using the low powerelectronic oscillator both during and between calibration cycles.

FIG. 3 illustrates an embodiment of a process for calibrating anoscillator during power-off mode. In some embodiments, process 300 isemployed at 210 of process 200. Process 300 starts at 302 at which theoscillator and reference frequency pulses are compared. In some cases,for example, the number of reference frequency pulses for a prescribednumber of oscillator pulses are counted and compared to the number ofreference frequency pulses that should occur for the prescribed numberof oscillator pulses for an ideal oscillator frequency. At 304, theoscillator frequency error, if any, is determined based on thecomparison of 302. In some cases, for example, 304 includes determiningwhether the actual oscillator frequency is higher or lower than theideal oscillator frequency. At 306, the oscillator is trimmed, ifapplicable, to at least in part correct the frequency error determinedat 304. At 308, it is determined whether to perform another iteration ofprocess 300. In some embodiments, a prescribed number of iterations ofprocess 300 are performed during a calibration cycle. In someembodiments, as many iterations needed to achieve a prescribed accuracyare performed during a calibration cycle. If it is determined at 308 toperform another iteration, process 300 continues at 302. If it isdetermined at 308 that the calibration cycle is complete, process 300ends.

In various embodiments, a crystal having any frequency may be employedas the reference frequency source for frequency generator 100. Forexample, in some embodiments, crystal 102 comprises a 14.318 MHzcrystal; and in such cases, frequency divider 110 may, for instance,comprise a fractional-n divider that divides by 437 for 131 pulses anddivides by 736 for 6 pulses to yield a 32.768 kHz output frequency. Insome embodiments, pulse comparator logic 120 includes logic to add orsubtract a (digital) offset to compensate for difficult to achieveratios between the reference frequency and an oscillator frequency,e.g., when comparing pulses of the oscillator and reference frequencyduring calibration.

In various embodiments, crystal 102 may have any type of cut, such asAT, BT, CT, SC, etc. Different performance characteristics areassociated with different cuts. For example, the orientation of a cutinfluences parameters such as frequency stability, thermalcharacteristics, aging characteristics, etc. The SC cut, for instance,has excellent aging properties and almost no frequency versustemperature error at higher temperatures. It may be advantageous toselect the cut of the crystal based at least in part on the intendedapplication since the performance characteristics associated with thecrystal are also exhibited by any frequencies generated using thecrystal frequency. Instead of a quartz crystal, any other high accuracyfrequency source may be employed in various embodiments such as a pulsepowered AlN, MEMs, tuned LC, or other resonator technology used forfrequency generation in electronic circuits.

In various embodiments, frequency generator 100 may be configured togenerate or output one or more frequencies of one or more values invarious operational modes. Generation of a 32.768 kHz output frequencyin power-off mode is described with respect to some of the examplesprovided herein because this frequency is typically used for RTCfunctions and chip beaconing during sleep wake-up. In other embodiments,frequency generator 100 may be configured and may include appropriatecircuitry to generate any one or more output frequencies duringpower-off mode in addition to and/or instead of 32.768 kHz. In somecases, frequency generator 100 may include a plurality of electronicoscillators and associated calibration circuitry to generate a pluralityof output frequencies using oscillators during power-off mode. Likewise,frequency generator 100 may be configured and may include appropriatecircuitry to generate any one or more output frequencies during power-onmode. In some cases, for example, frequency generator 100 may includeintegrated phase-locked loops or other equivalent circuitry to generateone or more 96 MHz DOT clocks, 48 MHz USB clocks, high frequency (e.g.,125 MHz-2 GHz) clocks, etc.

A system may comprise various time-related elements (e.g., delays,oscillators, power-on resets, sleep timers, etc.) and/orfrequency-related elements (e.g., auto-zero circuits, charge pumps,analog-to-digital converters, counters, etc.). In some embodiments, theoutputs of frequency generator 100 are branched out to one or morecomponents in a system that require an accurate time and/or frequencybase. In some embodiments, a single clock output of frequency generator100 may be employed to drive a plurality of clock inputs. In some cases,using frequency generator 100 for all or at least a portion of thetiming and/or frequency requirements of an associated system eliminatesor at least reduces error and mitigates the effects of process,temperature, and voltage variations from the system. In variousembodiments, frequency generator 100 may be configured to generate oroutput any combination, any number of copies, any multiple of, and/orany integer or fractional divide of one or more frequencies. Forexample, in some cases, frequency generator 100 may be configured togenerate or output multiple copies of the same low frequency (e.g.,32.768 kHz).

An error or systematic offset may be introduced into an output frequencyof frequency generator 100 due to various sources. In variousembodiments, an error introduced into an output frequency of frequencygenerator 100 may be corrected or tuned at any stage and in anyappropriate manner. Different errors may be introduced in differentmodes of operation (e.g., power-off and power-on) and in someembodiments may be independently tuned. In some cases, an error may beintroduced if synchronous noise is present at the calibration frequencyin power-off mode. In some cases, such an error may at least in part becorrected and/or averaged out by dithering the calibration frequency.For instance, calibration of an oscillator every 2.5 s can be ditheredsuch that on average the oscillator is calibrated every 2.5 s but actualcalibration periods comprise values around 2.5 s, such as 2.4-2.6 s, sothat, for example, a noise harmonic that also has a frequency of every2.5 s does not create an error offset. In some cases, referencefrequency source 102 may introduce an error, which if uncorrected,propagates to any output frequency generated based on the referencefrequency. In various embodiments, an error introduced by a referencefrequency source such as crystal 102 may be tuned in any appropriatemanner. In some embodiments, an error introduced by reference frequencysource 102 is externally tuned, e.g., using a resistor or capacitor, bystrapping pins or bond wires to the reference frequency source, etc. Insome embodiments, an error introduced by reference frequency source 102is internally tuned by frequency generator 100, e.g., with an integratedresistor, capacitor, transistor, etc., that is connected to referencefrequency source 102 via a serial or parallel communications bus. Insome embodiments, short-term jitter on the reference clock is at leastin part removed by combining a prescribed number of consecutive clockpulses or cycles into a single pulse to reduce or average out jitter andreconstructing the reference clock using such combined pulses. In somecases, an error may be introduced due to EMI (electromagneticinterference) between the output frequency or frequencies of frequencygenerator 100 and reference frequency source 102, especially if thereference frequency source is on the same die and/or in the samepackage. In some embodiments, frequency generator 100 integrates anyappropriate spread spectrum circuitry to reduce EMI (electromagneticinterference).

In some cases, pulse powering reference frequency source 102 creates acurrent spike that introduces an IR drop in an associated bond wireduring calibration. The voltage drop across the bond wire is presentduring calibration mode but disappears in free run mode when thereference frequency source is powered off As a result, the voltage dropacross the bond wire occurring during calibration manifests as aconstant frequency error in the calibrated oscillator. Any appropriatetechnique to tune or correct for this error may be employed in variousembodiments. In some cases, for example, the value of such a voltageoffset is determined using the following process. Reference frequencysource 102 is first powered-on at X current, and the frequency error inoscillator 108 at X current is measured and stored. Next, the currentdriving reference frequency source 102 is changed to X+Y, and thefrequency error in oscillator 108 at X+Y current is measured and stored.Finally, the current driving reference frequency source 102 is changedto Y current. The total current related offset error at Y current is thedifference between the oscillator frequency errors measured at X currentand X+Y current, i.e., errorX−errorX+Y, and is compensated bysubtracting this value from the oscillator.

Alternately enabling and disabling (i.e., pulse powering) a circuit canreduce the power consumption of the circuit on average, provided thatthe circuit is not needed some of the time. This condition is usuallynot true for a regulator circuit that provides a regulated power supplyto other circuits. Regulator accuracy is difficult to achieve when powerconsumption needs to be very low, e.g., less than 1 to or 100 nA. Insome embodiments, an alternately enabled and disabled (i.e., pulsepowered) bandgap regulator with a low power sample-and-hold circuit maybe employed to achieve both accurately regulated, continuously-availablesupply voltage and very low power consumption at the same time. A blockdiagram of an embodiment of the circuitry is provided in FIG. 4. In thisexample, total power consumption is on the order of 100 nA.

In various embodiments, higher levels of integration are achievable byintegrating frequency generator integrated circuit 100 with any otheranalog and/or digital circuitry. Integrating additional circuitry withfrequency generator 100 is often very cost effective, e.g., since higherintegration reduces overhead associated with handling, packaging, andtesting costs. In some embodiments, frequency generator 100 provides acore around which all or at least some of a system's timing and clockgeneration, distribution, and/or utilization can be included. In someembodiments, frequency generator 100 includes one or more integratedphase-locked loops to generate various output frequencies, e.g., asdescribed above with respect to the example of FIG. 1B. In someembodiments, real time clock/calendar circuitry is integrated intofrequency generator 100.

In some embodiments, backup power source 104 comprises a power-off orsleep mode battery such as an RTC or CMOS battery. Such backup batteriesare typically coin cell batteries of the CR, BR, or ML re-chargeabletype and are employed to power only critical circuit elements that arenever turned off, such as RTC circuitry. The CR and BR coin cellbatteries are not rechargeable. However, the ML coin cell battery isre-chargeable and finding widespread and growing use within theelectronic industry. From a system design perspective, the CR and BRbatteries may offer, for example, 10 years of battery life for a givenRTC circuit before needing replacement. The replacement issuenecessitates CR and BR batteries to be placed in a small battery holderthat allows easy replacement. A smaller ML battery may be selected forthe same application that gives only one year of battery life percharge. If the associated electronic product has a main batteryinstalled or is plugged into a wall socket at least once per year toallow the ML battery to be re-charged, there is no need to replace thebattery for the lifetime of the product. Thus, with the ML battery, asystem designer can elect a smaller form factor and the elimination of abattery holder but with the added penalty of a charger circuit and theuser limitation of periodically providing a charging power source.

An ML coin cell battery charger in its most basic form may quite simplybe implemented. In some embodiments, the charger may comprise a simplediode and current limiting resistor attached to a constant voltage. Forexample, if a 3.3 V power rail is available, a Schottky diode may beplaced in series with a 1000 Ω resistor and connected directly to the +node of the ML battery. In such cases, if the ML battery falls belowfull charge (e.g., ˜3 V), a small trickle charge (e.g., ˜100-250 μA)flows into the battery to re-charge it. If 3.3 V is not available, thecharger may be slightly more complex. For example, if a 5 V power railis available, there exists many ways to ensure that the battery is notover-voltaged or over-charged. For example, a simple transistor andresistors may be used to both current limit and shut-off charging whenthe voltage approaches the maximum limit of the ML battery. Varioustechniques to ensure that the ML battery is not over-voltaged orover-charged may include or make use of one or more of the followingfunctions or circuit elements: DC/DC converters; charge pumps; voltagedividers; LDO (low-dropout) regulators; voltage regulators; circuitscomprising operational amplifiers or voltage or current references; etc.

In some embodiments, backup battery management and/or charger circuitry(e.g., for managing and/or charging backup battery 104) is integratedinto frequency generator integrated circuit 100. The charger circuitrymay be appropriately configured for and/or employed with respect to anytype of for re-chargeable backup battery, such as ML coin cellbatteries, Lithium-ion batteries, etc. In many systems, a 32.768 kHzfrequency generator, real time clock logic, sleep timers, etc., arepowered from the same backup battery during power-off mode, and theircircuitry is often directly tied to each other and in close proximity ona printed circuit board. Thus, integrating such circuitry in frequencygenerator 100 is in many cases advantageous, e.g., for componentreduction, simplifying designer work by offering a pre-packaged andtested solution, reducing PCB (printed circuit board) area, etc.

In some embodiments, an external or integrated capacitor is employedwith respect to frequency generator 100 for facilitating quick turn-onof reference frequency source 102. As described, reference frequencysource 102 is periodically powered on in power-off mode to calibrateoscillator 108 of frequency generator 100. Fast turn-on of a highfrequency crystal in many cases requires a large current over a shorttime period. For example, a current on the order of 1-10 mA may beneeded for quick turn-on of a MHz crystal. However, a backup batterysuch as power source 104 may not be able to deliver such levels ofcurrent due to its internal resistance and UL (Underwriters Lab)current-limiting series resistor requirements. Such additional resistorsmay cause the voltage applied to the crystal to drop significantly,potentially affecting start-up performance. In some embodiments, asmoother start-up of reference frequency source 102 is achieved throughuse of a high value capacitor that is able to provide the necessarycurrent during the time the reference frequency source is ramping up andactive. During the time the reference frequency source is off, thecapacitor is slowly re-charged by the backup battery within its ratedspecification for continuous and pulse output current. In someembodiments, the value of the storing capacitor may be reduced bypumping a high voltage onto it. In some cases, for example, the value ofthe capacitor may be halved for every doubling of the applied voltage.

In some embodiments, a non-volatile memory is used in conjunction withfrequency generator 100. For example, the non-volatile memory may beemployed to trim for a particular frequency, better frequency errorversus environmental effects performance, and other clockingcharacteristics such as output buffer strength, calibration rate,synchronous or asynchronous calibration, etc. In various architectures,the non-volatile memory may be integrated on the same die as frequencygenerator 100, may be on a different die but part of the same package asfrequency generator 100, may be a part of an external package, may beconnected to another processor or SOC elsewhere in an associated system,etc. In various embodiments, the non-volatile memory may be used withrespect to frequency generator 100 for input frequency selection; outputfrequency selection; frequency trimming the input frequency; frequencytrimming the output frequency; frequency trimming the siliconoscillator; frequency trimming for offset, aging, temperature, powersupply voltage, integrated circuit stress, or any other electrical ormechanical phenomena that may impact an output frequency; etc. Invarious embodiments, the non-volatile memory may be a floating gate typeof re-programmable memory (e.g., flash, E2PROM, EPROM, etc.), one-timeprogrammable memory, a SONOS type memory, fuse, anti-fuse, bondingwires, IC metal mask, FIB, pseudo non-volatile memory (volatile memorywith battery back-up), etc. In some embodiments, the non-volatile memoryis not physically located on the integrated circuit of frequencygenerator 100 but loaded and/or adjusted from an external source. Insome cases, for example, the non-volatile memory is located next to aprocessor, FPGA, or controller used to manipulate frequency generator100.

In some embodiments, frequency generator 100 is packaged using WL-CSP orother chip scale packaging. In some embodiments, frequency generator 100is combined with reference frequency source 102 into a single package.In some cases, for example, frequency generator 100 is placed inside aquartz crystal package. A smaller PCB footprint is achievable by placingthe frequency generator die inside the quartz crystal package. Invarious embodiments, the crystal package may be of any size and/or typeincluding metal, ceramic, glass fritz, etc.

In various embodiments, frequency generator 100 may be used for anyappropriate application such as analog and digital watches and clocks;computers; portable media players; cameras; cell phones; media and/orstorage applications; notebook computers; netbooks; consumer,industrial, medical, automotive, communications, and/or militaryapplications; or any other application in which the benefits of lessaging, less cost, smaller size, less frequency error over environmentalconditions, etc., are valued. For example, traditionally a 32.768 kHzcrystal has been used for RTC circuitry in such applications. However,compared to a 32.768 kHz crystal, a 32.768 kHz frequency generated byfrequency generator 100 exhibits better performance in many respects.Consider, for example, a comparison of a 32.768 kHz tuning fork crystaland a 32.768 kHz frequency generated by frequency generator 100 using anAT cut crystal source 102. The 32.768 kHz frequency generated byfrequency generator 100 exhibits similar performance characteristics asthe AT cut crystal source. An AT cut crystal exhibits greatertemperature stability than a 32.768 kHz tuning fork crystal. A 32.768kHz tuning fork crystal is generally optimized for operation at 25° C.+/−5° C. and forms a parabolic frequency curve around this turnovertemperature; however, an AT cut crystal has a flatter frequency curvefrom −40° C. to +85° C. Thus, any watch or other electronic device thatincludes an AT cut crystal based frequency generator 100 will haveimproved performance in very hot and very cold environments without anyadditional trimming or other integrated circuit design techniques. Highperformance operation in extreme environments is a key feature for manydevices, including watches. Moreover, a small AT cut crystal has ahigher shock and vibration resistance compared to a 32.768 kHz tuningfork crystal. Thus, an AT cut crystal based frequency generator 100conveys higher immunity to shock and vibration without any additionalmaterial or mechanical design techniques. Furthermore, a small AT cutcrystal ages at a slower rate than a tuning fork crystal. Thus, an ATcut crystal based frequency generator 100 conveys better agingperformance.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a reference frequencysource for generating an output frequency when a main power source isavailable; and an oscillator for generating the output frequency whenthe main power source is not available, wherein the oscillator iscalibrated at prescribed calibration intervals to track the accuracy ofthe reference frequency source and wherein the reference frequencysource is powered-on during calibration intervals to calibrate theoscillator and otherwise powered-off when the main power source is notavailable.
 2. The system of claim 1, wherein generating the outputfrequency using the oscillator consumes less power than generating theoutput frequency using the reference frequency source.
 3. The system ofclaim 1, wherein the output frequency comprises a real time clockfrequency.
 4. The system of claim 1, wherein a reference frequency ofthe reference frequency source comprises a megahertz frequency
 5. Thesystem of claim 1, wherein the reference frequency source comprises acrystal.
 6. The system of claim 1, wherein the reference frequencysource is used to generate a plurality of frequencies including theoutput frequency when the main power source is available.
 7. The systemof claim 1, further comprising calibration circuitry for calibrating theoscillator.
 8. The system of claim 7, wherein the calibration circuitryis powered-on during calibration intervals and otherwise powered-offwhen the main power source is not available.
 9. The system of claim 1,wherein power is provided by a backup power source when the main powersource is not available.
 10. The system of claim 9, wherein the backuppower source comprises a battery.
 11. A method, comprising: generatingan output frequency using a reference frequency source when a main powersource is available; and generating the output frequency using anoscillator when the main power source is not available, wherein theoscillator is calibrated at prescribed calibration intervals to trackthe s accuracy of the reference frequency source and wherein thereference frequency source is only powered-on during calibrationintervals to calibrate the oscillator and otherwise powered-off when themain power source is not available.
 12. The method of claim 11, whereingenerating the output frequency using the oscillator consumes less powerthan generating the output frequency using the reference frequencysource.
 13. The method of claim 11, wherein the output frequencycomprises a real time clock frequency.
 14. The method of claim 11,wherein a reference frequency of the reference frequency sourcecomprises a megahertz frequency
 15. The method of claim 11, wherein thereference frequency source comprises a crystal.
 16. The method of claim11, wherein the reference frequency source is used to generate aplurality of frequencies including the output frequency when the mainpower source is available.
 17. The method of claim 11, furthercomprising calibrating the oscillator using calibration circuitry. 18.The method of claim 17, wherein the calibration circuitry is powered-onduring calibration intervals and otherwise powered-off when the mainpower source is not available.
 19. The method of claim 11, wherein poweris provided by a backup power source when the main power source is notavailable.
 20. The method of claim 19, wherein the backup power sourcecomprises a battery.